This invention relates to a correction circuit useful for correcting distortion of the raster of a television display device.
A switched pincushion correction circuit is described in U.S. Patent application Ser. No. 722,600, filed Sept. 13, 1976 for Peter Eduard Haferl and entitled "PINCUSHION CORRECTION CIRCUIT", in which a switched impedance is coupled in series with the horizontal deflection winding. The switch is rendered conductive or closed at a time during the second half of the horizontal retrace interval and remains conductive during the remainder of the horizontal retrace interval and for the entire succeeding trace interval. A switch control circuit changes the switching time during the horizontal retrace interval progressively during the vertical scanning interval. The ratio of the time during which the switch is ON or conductive to the OFF or nonconductive time during the retrace interval varies the effective or average impedance in series with the horizontal winding. The progressive variations in operating time therefore progressively vary the average impedance in series with the horizontal deflection winding at the vertical rate. To provide side pincushion correction, the average impedance in series with the horizontal deflection winding is relatively high at the top and bottom of the vertical scan and relatively low in the center of the vertical scan.
It is customary to energize other circuits of a television receiver from the horizontal deflection circuit. For example, a rectifier and filter are often transformer-coupled to an output of the horizontal deflection generator. A load such as a kinescope ultor or an audio or video amplifier is coupled to the filter and energized by power derived from the horizontal generator. The switched synchronous vertical deflection circuit (SSVD) as described in U.S. Patent application Ser. No. 595,809, filed July 14, 1975 for Peter Eduard Haferl and entitled "SWITCHED VERTICAL DEFLECTION SYSTEM", can also be energized by the horizontal deflection circuit. Such loads can be expected to vary during the normal course of their operation. The SSVD circuit has a load variation which varies in a periodic manner at the vertical deflection rate. The ultor and audio generator vary in consonance with the information content and are not periodic. Both types of load cause a reduction in the width of the horizontal retrace pulses with increasing load.
The load-dependent horizontal pulse-width reduction creates a raster side modulation in addition to the common side pincushion distortions. Loading of the horizontal deflection generator by an SSVD circuit creates periodic raster distortions having the general appearance of side pincushion type distortion, whereas loads having independent variations cause more unpredictable types of raster side distortion. When the aforementioned switched pincushion correction circuit is used this unwanted distortion occurs because the ON time relative to the OFF time of the switch changes when the horizontal retrace duration changes.